The present invention relates to devices using shape memory alloys (SMA), especially SMA rotary actuators for flexing aerospace control surfaces, and to switching mechanisms and methods for controlling the switching of SMAs between states using thermoelectric devices.
Shape memory alloys (SMA) form a group of metals that have interesting thermal and mechanical properties. If a SMA material such as NiTinol is deformed while in a martensitic state (low yield strength condition) and then heated to its transition temperature to reach an austenitic state, the SMA material will resume its original (undeformed) shape. The rate of return to the original shape depends upon the amount and rate of thermal energy applied to the component. When the SMA material is cooled, it will return to the martensitic state and shape. Properties of and information about shape memory alloys can be found at http://www.sma-inc.com/SMAPaper.html or http://www.SMA-mems.com/info.html, which we also incorporate by reference.
The application of xe2x80x98smart structuresxe2x80x99 to helicopter rotors has received widespread study in recent years, and is one thrust of the Shape Memory Alloy Consortium (SMAC) program, which Boeing leads. The SMAC program includes NiTinol fatigue/characterization studies, SMA actuator development, and ferromagnetic SMA material development (offering increased actuation speed). An SMA torsional (i.e. rotary) actuator of the present invention for rotocraft (i.e., helicopters or tilt rotors) retwists a rotocraft blade in flight, and results in a significant payload gain for the vehicle.
Distributed fibers of piezo material embedded in a composite blade can accomplish dynamic twisting. Twists of a degree or two are adequate to achieve dynamic control of vibration and acoustics. By laying the piezo fibers at xc2x145xc2x0 to the blade axis and actuating the piezo material along the fiber direction, the piezo strain twists the blade about 2xc2x0, either dynamically at control frequencies (for vibration and noise reduction) or statically (for some payload increase). Implementing this technology requires high quality, low cost piezo fibers and high voltage, high power efficient drive amplifiers.
A second method of dynamic control uses a flap, actuated by a piezo stack actuator mounted on the rotor spar. A few degrees of flap motion provide adequate dynamic control of the rotor blade. This method also achieves vibration or acoustic benefits.
One key to getting good response out of an SMA such as NiTinol is to have a good cooling path. The NiTinol needs to be kept as thin as possible, consistent with the load requirements. In one actuator design, we surrounded a NiTinol torque tube with a thin brass tube wrapped with Nichrome foil heater tape. We initially wound the heater tape directly on the torque tube, but discovered that the tape could not stand the large torsion actuation strain the tube undergoes. The axial windings of the tape were nonuniform to help keep the tube at a constant, desired temperature, but the attachments for the tube made it difficult to achieve even, constant heating. The housing carries the heat load from the passive torque tube when the actuator was unpowered. Small air gaps within the actuator impose significant thermal resistance, and grease was required between the NiTinol/brass and heater/housing. The performance measured for the device suggests that 500 watts of power are required for a 20-second response as a nominal value of the power requirement for a ⅙-scale actuator. Scaling laws based upon our tests indicate that simply scaling this design (to full scale) would result in a heavy actuator requiring large power to achieve a constant response time, or, alternatively, would sacrifice response time at reasonable power requirements.
U.S. Pat. No. 5,127,228 describes a xe2x80x98smart structurexe2x80x99 actuator device having an inner SMA (e.g., NiTinol) torque tube seated concentrically within an outer SMA torque tube. Ends of both tubes are mechanically restrained to an indexed position. Because one tube provides torque clockwise while the other tube provides counter-clockwise torque, the tubes are arranged in an opposing manner. Initially, both SMA members are in a martensitic state. A power supply supplies current to heaters that are connected to one of the tubes to control switching of the SMA between memory states. Heating causes rotation of the actuator in a clockwise or counter-clockwise rotational direction, as desired. The electrical energy passing through the heater(s) causes the SMA to which the heater is connected to transition from its martensitic to austenitic state, resulting in the rotation. Control of the electrical power to the heaters allows holding the actuator in a selected rotational position or allows rotation in either rotational direction.
To maintain a specific rotational position in a loaded condition, the ""228 device requires continuous electrical power to the heater elements for both SMA tubes. This shortcoming of the ""228 device adds significant system weight and complexity and requires excessive power. The ""228 device requires the addition of thermal insulation to isolate the tubes thermally or a sill design so that the heater for one tube does not heat the wrong tube and, thereby, unintentionally create an actuator malfunction.
Other known SMA rotary actuators use a single SMA member to produce the desired reciprocating rotation at desired intervals. These devices use the SMA member to provide rotation in one direction, while using a mechanical spring, a flexure, or another suitable restorative device to provide rotation of the actuator in the return direction. The force achievable with mechanical springs is limited. Large springs having adequate force add considerable weight and mass to the actuator mechanism, which lessens performance and restricts their implementation. The mechanical springs also deteriorate over time, which limits the reliability of the actuator.
A need exists for a SMA rotary actuator to provide any or all of the following properties: (1) controllable torque either in low or high amount, (2) operation in both rotational direction using switching of the state of a single SMA member, locking at a desired rotational position without requiring constant supply of power to heaters associated with the SMA members, or (3) return rotation without applying electrical power. Such a rotary actuator should also be capable of generating a significant torque over a large angle of rotation. A small size and low weight also is beneficial for an improved device. The actuator of the present invention addresses these needs.
The present invention relates to improved control and operating efficient for a shape memory alloy (SMA) device using a thermoelectric device to pump heat between the SMA and a heat sink. In one preferred embodiment, the SMA device is a rotary actuator (SMA torque tube) and the heat sink is another rotary actuator associated with the first in antagonistic relationship. Such a device is particularly suited for flexing control surfaces in aircraft, particularly a rotocraft blade. Each actuator preferably includes a locking mechanism to allow shutdown of power to the SMA devices.
A preferred rotary actuator of the present invention includes an actuator assembly having a torque tube formed of a shape memory alloy (SMA). A superelastic NiTinol return spring or another SMA torque tube in antagonistic relationship is associated with the torque tube to bias the torque tube toward an initial position. A torque tube heating element, especially a thermoelectric device, transfers heat from the SMA to switch it between states. Such switching causes rotation to an object connected to the actuator or generates a torque upon that object. In a preferred embodiment the torque causes blade twist in a rotocraft blade.
We can increase the payload of a rotocraft by changing the blade twist distribution between hover (where a highly twisted blade is desired) and forward flight (where a less twisted blade is better). The preferred actuator of the present invention uses a SMA because of its high strain capability. The SMA is attached near the root of the rotor. A passive torque tube transfers torque generated by the switching of the SMA between the rotor root and the tip. Multiple stages can be used to implement a nonlinear optimum twist distribution better than current systems. Possible combinations of SMA quasi-static and piezo dynamic control also appear attractive. The development of magnetically activated SMA materials (ferromagnetics) introduces the possibility of both dynamic and quasi-static control without need for hydraulic systems. A payload gain of about 900 pounds is achievable with a twist change in the blade of an order of 8 degrees. Such a twist requires an input torque of about 24,000 inch-pounds for a full-scale blade. A minimum weight system rotates an SMA actuator about 30-60 degrees to wrap up a passive torque tube that is torsionally soft (relative to the blade stiffness). The passive torque tube then transfers the torque to the blade to produce the desired 8 degree twist. This system also reduces the impact of blade torsional dynamics where design must avoid resonances close to any of the harmonics of the spin speed.
A preferred rotary actuator of the present invention rotates or torques an object to which it is attached. The actuator includes a shape memory alloy (SMA) torque tube, a superelastic return spring connected to the torque tube, and a torque tube heater, especially one or more thermoelectric devices, positioned in contact with or near the torque tube to control switching of the SMA between memory states. The torque tube is longitudinally twisted relative to the superelastic return spring. The heater causes the torque tube to enter an austenitic state in which the torque tube returns to an untwisted configuration. Cooling the torque tube causes it to return to a martensitic state, thereby allowing the superelastic return spring to retwist the torque tube to the initial rotational position.
A preferred locking assembly includes a housing, a lock socket having locked and unlocked positions, a spring positioned between the socket and the housing, and at least one SMA actuator rod stretched while in a martensitic state to interconnect between the housing and the lock socket to achieve the desired locking condition. A heating element is associated with the SMA rod to heat the rod to control its switching between states. The torque tube and the superelastic return spring are also usually located within the housing with either the torque tube or the superelastic return spring being connected to the housing. In a first position, the socket cannot be rotated separately relative to the connector. In the other socket position, however, the socket is rotatably disengaged from the connector. The spring continuously urges the socket into its first (locked) position. The actuator rod or rods are trained for memory in length. Activating the heating element causes the rod to shrink. Then, the socket moves to its second position to allow rotation of the connector and the attached superelastic return spring.
A helicopter blade twist rotation system of the present invention twists a helicopter blade having a blade root end, a blade tip end, and a longitudinal spar extending from the blade root end toward the blade tip. The preferred blade twist rotation system connects our SMA rotary actuator to the blade spar near the blade root, and connects a torsionally flexible passive torque tube to the blade spar. The passive torque tube is connected to the rotary actuator and to the blade near to the blade tip. Rotation of the rotary actuator rotates the torque tube with rotation of the proximal end being greater than rotation of the torque tube distal end. Such twisting of the blade can produce a greater than 80% reduction in vibration, a 5-10 dB reduction in BVI acoustic noise while providing the payload increase (about 15% of the rated capacity for the V-22).